In recent years, for the sake of weight reduction of an automobile leading to improvement in fuel efficiency, weight reduction of a steel sheet to be used by increasing the strength of a steel sheet has been endeavored. However, when the strength of a steel sheet to be used is increased, there occurs a problem of occurrence of scoring or steel sheet fracture during molding, or instability of the shape of a molded item due to a spring-back phenomenon.
As a technology for producing a high strength component, there is a method by which the strength is increased after press molding, instead of pressing a high strength steel sheet. An example of the same is hot stamp molding. Hot stamp molding is a method by which a steel sheet to be molded is heated in advance for facilitating molding, and subjected to press molding keeping the high temperature as also described in Patent Literature 1, and 2. As a molding material therefor, a quenchable steel grade is selected, and a higher strength is achieved by quenching on the occasion of cooling after pressing. By this procedure, the strength of a steel sheet can be enhanced at the same time as press molding without conducting a separate heat treatment step for strength increase after press molding.
However, since hot stamp molding is a molding method by which a heated steel sheet is processed, formation of a Fe scale by surface oxidation of the steel sheet is unavoidable. Even in a case in which a steel sheet is heated in a non-oxidizing atmosphere, when the sheet is taken out from a heating furnace for press molding, a Fe scale is formed on a surface due to exposure to the air. Further, heating in such a non-oxidizing atmosphere is costly.
In a case in which a Fe scale is formed on a steel sheet surface during heating, the Fe scale may be peeled off during pressing to stick to a mold, so as to develop such a problem that the productivity of pressing may be impaired, or the Fe scale remains on a product after pressing to disfeature the appearance. Further, in a case in which such an oxide film remains, since a Fe scale on a surface of a molded item is poor in adhesiveness, when a conversion treatment and painting are performed on a molded item without removing the scale, a problem in paint adhesiveness will be developed.
Therefore, ordinarily a Fe scale is removed by applying a sandblasting treatment or a shotblasting treatment after hot stamping, and thereafter a conversion treatment or painting is carried out as described in Patent Literature 3. However, such a blasting treatment is troublesome, and impairs remarkably the productivity of hot stamping. Further, a strain may be generated in a molded item.
Meanwhile, a technology, by which hot stamping is conducted on a zinc-based coated steel sheet or an aluminum coated steel sheet, while suppressing Fe scale generation, has been disclosure in Patent Literature 4 to 6. Further, a technology for preforming a hot press on a coated steel sheet is also disclosed in Patent Literature 7 to 10.
Further, a method for producing a zinc-based coated steel sheet is disclosed in Patent Literature 11 and 12.    Patent Literature 1: Japanese Patent Application Laid-Open (JP-A) No. H07-116900    Patent Literature 2: JP-A No. 2002-102980    Patent Literature 3: JP-A No. 2003-2058    Patent Literature 4: JP-A No. 2000-38640    Patent Literature 5: JP-A No. 2001-353548    Patent Literature 6: JP-A No. 2003-126921    Patent Literature 7: JP-A No. 2011-202205    Patent Literature 8: JP-A No. 2012-233249    Patent Literature 9: JP-A No. 2005-74464    Patent Literature 10: JP-A No. 2003-126921    Patent Literature 11: JP-A No. H04-191354    Patent Literature 12: JP-A No. 2012-17495